Techniques have already been proposed to accurately control the idling speed of an engine in correspondence with the speed and temperature of the engine and the operation states of accessories installed in association with the engine, such as an air conditioner and a power steering, by arranging an idling speed control valve [hereinafter called an "ISC (idle speed control) valve"] in a throttle bypass passage and controlling the opening of the ISC valve on the basis of signals indicative of the respective operation states.
According to such a technique, target openings (or target engine speeds) corresponding to respective engine temperatures are set in advance. The ISC valve is first controlled to a target opening (or at an opening which makes it possible to control the engine speed to a target engine speed). When a change occurs in the load of the air conditioner or the like, the opening of the ISC valve is controlled further by a degree corresponding to the kind of the change in the load so that any change in the engine speed due to the change in the load can be compensated.
In this technique, the ISC valve is designed to permit setting the quantity of inducted air in a wide range from the setting of a large quantity of inducted air required at the time of a cold engine state to the setting of a small quantity of inducted area needed at the time of a hot engine state because there is a large difference between the quantity of air to be inducted through a throttle bypass passage required at the time of the cold engine state and the quantity of air to be inducted through the throttle bypass passage needed at the time of the hot engine state. The above technique therefore involves the problem that the idling speed becomes higher at the time of a hot state than it is needed if any trouble occurs on the ISC valve or its drive circuit or the like and the ISC valve is hence fixed at the setting for the large quantity of inducted air for the time of the cold state.
As is disclosed in Japanese Patent Application Laid-Open (Kokai) No. SHO 64-87843 or the like, a technique has been proposed accordingly to arrange a valve element (limiter), which operates responsive to the temperature of the engine, in the bypass passage in series with the ISC valve so that at the time of a hot state, especially, the maximum quantity of air inducted and flowing through the bypass passage at the time of the hot state is limited to prevent any unnecessary increase in the idling speed.
The arrangement of such a limiter in the bypass passage is however accompanied by the problem that the flow rate so controlled is affected by the opening of the limiter, for example, as shown in FIG. 11 especially when the opening of the ISC valve is relatively large.
Described specifically, consider, for example, the state that the engine temperature is relatively low (in this case, the maximum bypass flow rate by the limiter is relatively high) and most of accessories to the engine are not operated (State A) and the state that the engine temperature is relatively high (in this case, the maximum bypass flow rate by the limiter is relatively low) and most of the accessories other than a specific accessory are operated (State B). Assume that in each of the states, the opening of the ISC valve is substantially the same in a relatively high opening range.
Also assume that operation of the specific accessory was started from this state and the opening of the ISC valve has been increased by a degree preset corresponding to the accessory in both the states. Although the opening has been increased by the same degree in both State A and State B, the influence of the limiter is smaller in State A so that the quantity of air to be inducted is increased by a greater degree whereas the influence of the limiter is relatively greater in State B so that the quantity of air to be inducted is increased by a smaller degree. Consequently, despite the occurrence of the same load by the same accessory in both the states, the increase in the quantity of air to be inducted for the compensation of the load is affected by the engine temperature and is hence not performed by the same degree.
Further, consider the above situation under the same temperature condition. The influence by the limiter is small when the opening of the ISC valve is relatively small but, when the above opening becomes relatively large, the degree of influence of the limitation to the flow rate by the limiter increases. Here again, there is the problem that air to be inducted cannot be obtained precisely in a quantity inherently required in accordance with the degree of influence by the limier on the basis of the opening of the ISC valve if the change in the opening of the ISC valve is simply set at a fixed value in correspondence to a change in a specific operation state typified by a change in the operation of the accessory.
The present invention has been completed with such problems in view, and has as an object the provision of a control system for the quantity of air to be inducted into an engine so that, where an ISC valve and a limiter are arranged in series with each other in a bypass passage, the opening of the ISC valve is controlled while applying a correction in accordance with the state of temperature of the engine, thereby making it possible to independently make a change to the quantity of air inducted for the compensation of a load without being affected by the engine temperature and also to accurately obtain inducted air in a quantity inherently required in accordance with the degree of influence of the limiter which varies based on the opening of the ISC valve.